tdesktop: 1.8.15 -> 1.9.3

The GYP build is now deprecated [0].
This results in a large number of changes and many custom Nix patches
aren't required anymore (and probably haven't been required for quite
some time now, the derivation got a bit outdated...).

A lot of the changes in this commit are based on the changes of the
Arch package [1][2] (which our package is based upon).

Rough overview of the changes:
- gcc9 -> ninja (optional but let's follow Arch here)
- Dropped GYP, only CMake now
  - But: Python is still required
- fetchFromGitHub -> fetchurl (optional?)
- Apply all Arch patches and remove old patches
  - Requires one new patch for range-v3
- New dependencies: enchant2, lz4, xxHash
  - TODO: Plus a few new dependencies that shouldn't be required
- Cleanup: Irrelevant flags (e.g. GYP_DEFINES) and patches (e.g. sed)
  - Simplifies quite a few things :)
- Some additional documentation and TODOs

Co-Authored-By: Jan Tojnar <jtojnar@gmail.com>

[0]: https://github.com/telegramdesktop/tdesktop/issues/7001
[1]: https://git.archlinux.org/svntogit/community.git/commit/?id=23eff2b1ef7435441e93120618ca899f0b0e7e61
[2]: https://git.archlinux.org/svntogit/community.git/commit/?id=6a19e949724b2e2bfcdcf2081111ecd46108e449